1. Field of the Invention
The present invention relates to a differentially-fed slot antenna with which a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range, is transmitted or received.
2. Description of the Related Art
In recent years, drastic improvements in the characteristics of silicon-type transistors have led to an accelerated trend where compound semiconductor transistors are being replaced by silicon-type transistors not only in digital circuitry but also in analog high-frequency circuitry, and where analog high-frequency circuitry and digital baseband circuitry are being made into a single chip. As a result of this, single-ended circuits (which have been in the mainstream of high-frequency circuits) are being replaced by differential signal circuits which undergo a balanced operation of signals of positive and negative signs. This is because a differential signal circuit provides advantages such as drastic reduction in unwanted radiation, obtainment of good circuit characteristics under conditions which do not allow an infinite area of ground conductor to be disposed within a mobile terminal device, and so on.
The individual circuit elements in a differential signal circuit need to operate under a balance. Silicon-type transistors do not have much variation in characteristics, and make it possible to maintain a differential balance between signals. Another reason is that differential lines are also preferable for avoiding the loss that is associated with the silicon substrate itself. This has resulted in a strong desire for high-frequency devices, such as antennas and filters, to support differential signal feeding while maintaining the high high-frequency characteristics that have been established in single-ended circuits.
FIG. 26A shows a schematic see-through view as seen from the upper face. FIG. 26B shows a cross-sectional structural diagram taken along line A1-A2 in the figure. This is a ½ wavelength slot antenna (Conventional Example 1) which is fed through a single-ended line 103.
On a ground conductor surface 105 which is formed on the rear face of a dielectric substrate 101, a slot resonator 111A having a slot length Ls corresponding to a ½ effective wavelength is formed. In order to satisfy the input matching conditions, a distance Lm from an open-end point 113 of the single-ended line 103 until intersecting the slot 111A is set to a ¼ effective wavelength at the operating frequency. The slot resonator 111A is obtained by removing the conductor completely across the thickness direction in a partial region of the ground conductor surface 105.
As shown in the figure, a coordinate system is defined in which a direction that is parallel to a transmission direction in the feed line is the X axis and the plane of the dielectric substrate is the XY plane.
Typical examples of radiation directivity characteristics of Conventional Example 1 are shown in FIGS. 27A and 27B. FIG. 27A shows a radiation directivity in the YZ plane, whereas FIG. 27B shows a radiation directivity in the XZ plane. As is clear from these figures, Conventional Example 1 provides radiation directivity characteristics that exhibit a maximum gain in the ±Z direction. Null characteristics are obtained in the ±X direction, and even in the ±Y direction, a gain reduction effect of about 10 dB relative to the main beam direction is obtained.
U.S. Pat. No. 6,765,450 (hereinafter “Patent Document 1”) discloses a circuit structure in which the aforementioned slot structure is disposed immediately under a differential feed line so as to be orthogonal to the transmission direction (Conventional Example 2). That is, the circuit construction of Patent Document 1 is a construction in which the circuit for feeding the slot resonator is changed from a single-ended line to a differential feed line.
The construction described in Patent Document 1 has an objective to realize a function of selectively reflecting only an unwanted in-phase signal that has been unintendedly superposed on a differential signal. As is clear from this objective, the circuit structure disclosed in Patent Document 1 does not have a function of radiating a differential signal into free space.
FIGS. 28A and 28B schematically illustrate field distributions occurring in a ½ wavelength slot resonator in the cases where it is fed through a single-ended line and a differential feed line, respectively.
In the case of the slot being fed through a single-ended line, electric fields 201 are distributed along the slot width direction so that a minimum intensity exists at both ends and a maximum intensity exists in the central portion. On the other hand, in the case of the slot being fed through a differential feed line, electric fields 201a which occur in the slot due to a voltage of the positive sign and electric fields 201b which occur in the slot due to a voltage of the negative sign are at an equal intensity and have vectors in opposite directions. Thus, in total, both electric fields cancel out each other, so that no resonance phenomenon occurs. Therefore, even the ½ wavelength slot resonator is fed through a differential feed line, efficient radiation of electromagnetic waves would be impossible according to principles. Therefore, as compared to the case of feeding via a single-ended line, it is not easy to realize antenna characteristics by allowing a differential feed line to couple to a ½ wavelength slot resonator.
In general, in order to efficiently radiate electromagnetic waves from a differential transmission circuit, no slot resonator is used. Rather, a method is employed in which the interspace between two signal lines of a differential feed line is gradually increased to realize a operation as a dipole antenna (Conventional Example 3).
FIG. 29A shows a perspective schematic see-through view of a differentially-fed strip antenna; FIG. 29B shows an upper schematic view thereof; and FIG. 29C shows a lower schematic view thereof. In FIGS. 29A to 29C, coordinate axes are set similarly to FIG. 26.
In a differentially-fed strip antenna, the line interspace of a differential feed line 103c which is formed on the upper face of a dielectric substrate 101 has a tapered increase at the ends. At the rear face side of the dielectric substrate 101, a ground conductor 105 is formed in a region 115a which is closer to the input terminal, whereas no ground conductor is formed in a region 115b lying immediately under the ends of the differential feed line 103c. 
Typical examples of radiation directivity characteristics of Conventional Example 3 are shown in FIGS. 30A and 30B. FIG. 30A shows radiation directivity characteristics in the YZ plane, whereas FIG. 30B shows radiation directivity characteristics in the XZ plane.
As is clear from these figures, in Conventional Example 3, the main beam direction is the ±X direction, and Conventional Example 3 exhibits radiation characteristics with a broad half-width distributed over the XZ plane. According to principles, no radiation gain in the ±Y direction is obtained in Conventional Example 3. Radiation in the minus X direction can be suppressed due to the reflection from the ground conductor 105.
Japanese Laid-Open Patent Publication No. 2004-274757 (hereinafter “Patent Document 2”) discloses a variable slot antenna which is fed through a single-ended line. FIG. 1 of Patent Document 2 is shown herein as FIG. 31.
This construction is similar to Conventional Example 1 in that a ½ wavelength slot resonator 5 which is formed on the substrate rear face is fed through a single-ended line 6 which is disposed on the front face of the dielectric substrate 10. However, at the leading end of the ½ wavelength slot resonator 5 being fed, a plurality of ½ wavelength slot resonators 1, 2, 3, and 4 are further provided for selective connection, thus realizing highly-free slot resonator positioning. It is described that changing the slot resonator positioning realizes a function of changing the main beam direction of electromagnetic waves (Conventional Example 4). (Non-Patent Document 1: Artech House Publishers “Microstrip Antenna Design Handbook” pp. 441-pp. 443 2001)
Conventional differentially-fed antennas, slot antennas, and variable antennas have the following problems associated with their principles.
Firstly, in Conventional Example 1, the main beam can only be directed in the ±Z axis direction, and it is difficult to direct the main beam direction in the ±Y axis direction or the ±X axis direction. What is more, since differential feeding is not yet supported, it is necessary to employ a balun circuit for feed signal conversion, thus resulting in the problems of increased elements, hindrance of integration, and the like.
Secondly, the ½ wavelength slot resonator of Conventional Example 2, in which feeding via a single-ended line is merely replaced with feeding via a differential feed line, can only acquire non-radiation characteristics. Thus, it is difficult to obtain an efficient antenna operation.
Thirdly, with Conventional Example 3, it is difficult to direct the main beam in the ±Y axis direction. Note that bending the feed line in order to deflect the main beam direction is not an available solution in Conventional Example 3 because, if the differential line is bent, the reflection of an unwanted in-phase signal will occur due to a phase difference between the two wiring lines at the bent portion. As an antenna for a mobile terminal device to be used in an indoor environment, it is highly unpreferable that the main beam cannot be directed in a certain direction.
Fourthly, the radiation characteristics of Conventional Example 3 have a broad half-width, which makes it difficult to avoid deterioration in quality of communications. For example, if a desired signal comes in the Z axis direction, the reception intensity of any unwanted signal that comes in the +X direction will not be suppressed. Thus, it is very difficult to avoid serious multipath problems which may occur when performing high-speed communications in an indoor environment with a lot of signal returns, and maintain the quality of communications in a situation where a lot of interference waves may arrive.
Fifthly, as in the aforementioned fourth problem, it is also difficult in Conventional Example 4 to prevent the quality of communications from being unfavorably affected by an unwanted signal coming in a direction which is different from the direction in which a desired signal arrives. In other words, even if the main beam direction is controllable, there is still a problem of inadequate suppression of interference waves. Of course, as in the aforementioned first problem, differential feeding is not yet supported.
In summary, by using any of the conventional techniques, it is impossible to realize a variable antenna which simultaneously solves the following three problems: 1) affinity with differential feed circuitry; 2) ability to switch the main beam direction within a broad range of solid angles; and 3) suppression of interference waves coming in any direction other than the main beam direction.